The present invention relates to an intake passage construction for an internal combustion engine having a two-barrel carburetor.
There have been known internal combustion engines having two-barrel carburetors including a primary intake passage for supplying an air-fuel mixture when the engine operates under a full range of loads and a secondary intake passage for supplying an air-fuel mixture when the engine operates under relatively high loads. Such an internal combustion engine has a secondary slow fuel passage having fuel supply ports opening into the secondary intake passage for smoothing transient engine operation when the secondary intake system comes into operation in addition to the primary intake system. In a practical range of engine operation, however, only the primary intake passage operates and no air-fuel mixture flows through the secondary intake passage. During such operation, fuel in the secondary slow fuel passage is evaporated with heat to thereby cause vapor lock in the secondary slow fuel passage. The vapor lock tends to prevent fuel from being supplied into the secondary intake passage when the latter is required to operate, resulting in hesitation and hence poor drivability. To solve this problem, there has been proposed to supply a portion of fuel from the secondary slow fuel passage into the primary intake passage or induction pipe at all times. With such an arrangement, however, large amounts of fuel are likely to be drawn off intermittently from the secondary slow fuel passage under the influences of intake pulsations and high vacuums developed in the primary intake passage when the engine is subjected to small loads. Fuel combustion thus becomes unstable in a range in which a small amount of fuel is to flow into the engine, with resulting poor drivability.